The use of bitumen in the manufacture of materials for highway and industrial applications has been known for a long time: bitumen is the main hydrocarbon binder used in the field of road construction or civil engineering. To be able to be used as a binder in these different applications, the bitumen must have certain mechanical properties, and in particular elastic or cohesive properties. The mechanical properties of the bituminous compositions are determined by standardized tests of the different mechanical characteristics such as the softening point, the penetrability and the rheological characteristics in defined traction.
In general, the conventional bitumens do not simultaneously have all of the required qualities and it has been known for a long time that the addition of various polymers to these conventional bitumens makes it possible to favourably modify the mechanical properties of the latter and to form bitumen-polymer compositions having improved mechanical qualities compared with those of the bitumens alone. The polymers capable of being added to the bitumens are most often elastomers. Among the polymers added to bitumens, random or block copolymers of an aromatic monovinyl hydrocarbon and a conjugated diene and in particular of styrene and butadiene or of styrene and isoprene are particularly effective as they dissolve very easily in the bitumens and confer upon them excellent mechanical and dynamic properties and in particular very good viscoelastic properties.
It is also known that the stability of the bitumen/polymer compositions can be improved by chemical coupling of the polymer with the bitumen, this improvement moreover making it possible to extend the field of use of the bitumen-polymer compositions. The chemical coupling of the polymer with the bitumen consists of cross-linking the polymer by means of a cross-linking agent, in a standard fashion, a sulphur-donor compound. Bitumen-polymer compositions for which a random or block copolymer of styrene and a conjugated diene such as butadiene or isoprene is coupled with the bitumen can be prepared using the processes described in the citations FR-A-2376188, FR-A-2429241, FR-A-2528439 and EP-A-0360656. In these processes, the source of sulphur consists of chemically non-bound sulphur (FR-A-2376188 and FR-A-2429241), in a polysulphide (FR-A-2528439) or in a sulphur-donor vulcanization accelerator used alone or in combination with chemically non-bound sulphur and/or a polysulphide or a non-sulphur-donor vulcanization accelerator (EP-A-0360656).
The cross-linked bitumen/polymer compositions thus obtained are known by the acronym “PmB”, for Polymer-modified Bitumen or “PmA” for Polymer-modified Asphalt. The cross-linking of the bitumen/polymer compositions confers upon them very good properties in terms of storage stability, cohesion, elongation capacity and resistance to ageing.
However, the use of sulphur-donor cross-linking agent, in particular, the use of elemental sulphur for the cross-linking step leads to a significant emission of hydrogen sulphide, denoted H2S, during the PmB production process. Hydrogen sulphide (H2S) is a colourless and toxic gas, having a characteristic odour at a very low concentration. In the PmB production units, the concentration of H2S released during the manufacture of a cross-linked bitumen/polymer composition is particularly significant. The release of H2S is much greater for the cross-linked bitumen/polymer compositions than for bitumen bases devoid of cross-linking agent. For reasons of safety and because of environmental constraints, the reduction or even the elimination of hydrogen sulphide emissions during the production of PmB constitutes a crucial industrial challenge.
Solutions have been proposed in the literature for reducing hydrogen sulphide emissions during the manufacture of PmB. In particular, the addition of an agent capable of scavenging hydrogen sulphide (H2S) during the cross-linking of bitumen/polymer compositions has been proposed in order to find a solution to this drawback. By way of example, there may be mentioned the organic or inorganic metal salts described in the international application WO2005065177 as agents capable of scavenging H2S. The process implemented for reducing the release of H2S consists of introducing an organic or inorganic metal salt dissolved in bitumen into a reactor containing a previously mixed bitumen/polymer composition. The metal salt added in mass into the reactor, is then stirred in the reactor during the cross-linking reaction.